The invention relates to a method for operating a radio communication system which uses OFDM, and to a corresponding radio communication system, a corresponding sending station and a corresponding receiving station.
In radio communication systems which use OFDM (Orthogonal Frequency Division Multiplex), several subcarriers are available for data transmission during the duration of one OFDM symbol. In this context, one OFDM symbol can in each case be modulated onto each subcarrier onto the respective carrier frequency and sent by a sending station, for example a base station or a subscriber station. An OFDM symbol can contain useful data and/or signaling data. Furthermore, an OFDM symbol can also be used as reference symbol and be used by a receiver for estimating the transmission channel. During the period of an OFDM symbol, one or more subcarriers can be used for sending reference symbols.
If a sending station sends a reference symbol on a carrier frequency to a receiving station for channel estimation, the reception of the reference symbol is disturbed by interferences. These are especially interferences with reference symbols which are sent by the sending station in adjacent sectors on the carrier frequency or interferences with reference symbols which other sending stations located within radio range of the receiving station are sending on the carrier frequency (inter-cell interference). To reduce these interferences and to improve the quality of the channel estimation, it was proposed in standardization meetings of the 3GPP LTE (3rd Generation Partnership Project Long Term Evolution) to distinguish reference symbols which are sent by a sending station in different sectors, by a code, i.e. a CDM (Code Division Multiplex) component. Interferences with reference symbols of other sending stations are intended to be reduced with respect to the subcarriers used for sending the reference symbols, i.e. the respective carrier frequencies, by a frequency hopping method by each sending station using a different hopping sequence.
It is also proposed to send the reference symbols with a greater transmitting power than OFDM symbols sent simultaneously on other subcarriers, i.e. on other carrier frequencies, which contain useful and/or signaling data. Increasing the transmitting power has the effect that the reference symbols are received with a better signal to noise ratio (SNR) than the OFDM symbols with useful and/or signaling data. However, this has the disadvantage that interferences between adjacent radio cells of a different sending station are increased.